The present invention relates to an input protective circuit for a semiconductor device. More particularly, it relates to an input protective circuit for preventing the breakdown of the insulating layer of a device having MIS type structure such as MIS field-effect transistor, MIS diode, MIS capacitor etc. and an integrated circuit including the device.
In the conventional MIS type device and integrated circuit including the device, surge-noise voltage having a high peak value is sometimes applied from the outside into the input terminal such as the gate when the device is being prepared and handled whereby the gate-insulating layer may be broken down. Recently, integrated circuits are made more compact and designed to operate at a quick speed whereby the thickness of the gate insulating layer of the device is smaller. For example, the surge withstand voltage of silicon dioxide layer having a thickness in the range between 400 .ANG. and 500 .ANG. is almost 40 to 50 volts. Therefore, an input protective circuit assuring the normal operation even in the application of lower surge voltage and the operation characteristic of the device is needed.
FIG. 1 shows an example of the conventional N-channel type input protective circuit.
In the figure, the reference numeral (1) designates a MIS transistor, (2) designates an integrated circuit including the MIS transistor (1), (3) designates an external input terminal, (4) designates a resistor formed, for example, by an N type conductive diffusion region, (5) designates a rectifying diode formed between the N type region and a P type semiconductor substrate, (6) designates an electrostatic capacitance between a node (7) and the ground in an equivalent circuit. The node (7) is also used as an input terminal of the MIS transistor (1) and the integrated circuit (2).
The operation of the protective circuit will be described. When a positive surge voltage is applied to the external input terminal (3), surge current is reduced by the resistor (4) and the node (7) is clamped by the reverse breakdown voltage V.sub.BD of the diode (5).
On the other hand, when a negative surge voltage is applied to the external input terminal (3), the node (7) is clamped by the negative voltage (usually -0.6 to -0.7 volt) which is lower than the ground potential for the forward voltage drop in the diode (5).
The conventional input protective circuit having the structure described above has the following disadvantage. The external input terminal (3) is usually in a high input impedance state, that is, the diode is normally in non-conductive state. Therefore, when a positive surge voltage is applied to the external input terminal, the potential of the node (7) increases to the reverse breakdown voltage of the diode (5). The reverse breakdown voltage of the diode (5) is relatively high voltage such as several tens volt in a normally-used impurity concentration of the semiconductor substrate on which an integrated circuit is formed. It is difficult to control the breakdown voltage. It is, therefore, difficult to prevent the breakdown of the insulting layer of the integrated circuit in which the thickness of the gate insulating layer of the internal circuit is in the range between 400 .ANG. and 500 .ANG..